1. Field of the Invention
The present invention relates to a cellulose acylate film, a polarizing plate using the same, and a liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices have found wide range of uses for monitors of personal computers or portable devices, and television sets from various advantages of low voltage/low consumption power, capabilities of the reduction of size/the reduction of the film thickness, and the like. For such liquid crystal display devices, various modes have been proposed according to the orientation states of liquid crystal molecules in each liquid crystal cell. However, conventionally, the TN mode in which liquid crystal molecules are oriented to be twisted at about 90° from the lower substrate toward the upper substrate of a liquid crystal cell has been the main stream.
Generally, a liquid crystal display device includes a liquid crystal cell, an optical compensation sheet, and a polarizing film. The optical compensation sheet is used in order to eliminate image coloration or enlarge the viewing angle. A stretched birefringent film or a film obtained by coating a liquid crystal on a transparent film is used. For example, Japanese Patent No. 2587398 discloses the following technology: a discotic liquid crystal compound is coated on a triacetyl cellulose film, and oriented and fixed, resulting in an optical compensation sheet, and the optical compensation sheet is applied to a TN mode liquid crystal cell, thereby to enlarge the viewing angle. However, a stringent requirement is imposed on the viewing angle dependency of a liquid crystal display device for use in a television set which is supposed to be seen through the large screen at various angles. Thus, even with the foregoing technique, the requirement cannot be satisfied. For this reason, a study has been conducted on the liquid crystal display devices of the IPS (In-Plane Switching) mode, the OCB (Optically Compensatory Bend) mode, the VA (Vertically Aligned) mode, and other modes than the TN mode. Particularly, the VA mode has received attention as for use in a liquid crystal display device for a television set because of its high contrast, and a relatively high manufacturing yield.
Incidentally, the cellulose acylate film has a feature of higher optical isotropy (low retardation value) as compared with other polymer films. Therefore, for the purpose requiring the optical isotropy, for example, for the components of a polarizing plate, a cellulose acylate film is commonly used.
On the other hand, the optical compensation sheet (phase film) of a liquid crystal display device is, conversely, required to have the optical anisotropy (high retardation value). Particularly, the optical compensation sheet for the VA mode is required to have a in-plane retardation (Re) of 30 to 200 nm, and a retardation in a thickness-direction of 70 to 400 nm. Therefore, commonly used optical compensation sheets have been synthetic polymer films having a high retardation value such as a polycarbonate film and a polysulfone film.
As described above, in the technical field of optical materials, there has been the following general principle: when a polymer film is required to have an optical anisotropy (high retardation value), a synthetic polymer film is used; whereas, when the film is required to have an optical isotropy (low retardation value), a cellulose acylate film is used.
EP 0911656A2 proposes a cellulose acetate film having a high retardation value which can be also used for the purpose requiring the optical anisotropy, which disproves the conventional general principle. In this proposal, in order to achieve a high retardation value with cellulose triacetate, an aromatic compound having at least two aromatic rings is added, and a stretching treatment is carried out. It is generally known that cellulose triacetate is a polymer material which is difficult to stretch, and is difficult to increase in birefringent index. However, simultaneous orientation of an additive with a stretching treatment enables the increase in birefringent index. Thus, a high retardation value is implemented. This film can also serve as a protective film of a polarizing plate, and hence has an advantage in capability of providing a low-cost and thin liquid crystal display device.
JP-A-2002-71957 discloses an optical film containing a cellulose ester which has an acyl group having 2 to 4 carbon atoms as a substituent, and in which the expressions 2.0≦A+B≦3.0 and A<2.4, where A represents the substitution degree of an acetyl group, and B represents the substitution degree of a propionyl group or a butyryl group, are simultaneously satisfied, characterized in that 0.0005≦Nx−Ny≦0.0050 is further satisfied, where Nx represents the refractive index in the direction of the slow axis, and Ny represents the refractive index in the direction of the fast axis, at a wavelength of 590 nm.
JP-A-2004-277581 discloses a cellulose ester film containing a cellulose ester resin of which the degree of substitution of a hydroxyl group simultaneously satisfies the following expressions (1) and (2), and containing an ultraviolet absorbing polymer including and derived from an ultraviolet absorbing monomer of a specific structure in an amount of 1 to 20 parts by mass per 100 parts by mass of the cellulose ester resin, and characterized by being stretched so that the retardation value in the in-plane direction R0 is 20 to 100 nm, and the retardation value in the thickness direction Rt is 70 to 300 nm:2.4≦A+B≦2.8  Expression (1)1.4≦A≦2.0  Expression (2)[where in the formula, A represents the substitution degree of an acetyl group; and B, the substitution degree of an acyl group having 3 or 4 carbon atoms.]
JP-A-2003-270442 discloses a polarizing plate for use in a VA mode liquid crystal display device, the polarizing plate having a polarizing film, and an optically biaxial mixed fatty acid cellulose ester film, characterized in that the optically biaxial mixed fatty acid cellulose ester film is disposed between the liquid crystal cell and the polarizing film